Next project: Adcom GDA-700

[belgianbrain]

Rather than continuing to pollute the GDA-600 thread, I'm starting a new one for my new project.

Having been absolutely stunned by the sound quality of my modded GDA-600, I figured I need to try a GDA-700.

Just browsing the schematics, a few preliminary observations:

1. The balanced digital input now looks to be truly balanced, delivering a balanced signal all the way to the receiver chip.
2. The balanced analog output is somewhat fake. The PCM1702 chips seem to not have differential outputs, so the single ended outputs are taken through I/V and filter opamp stages, and a separate opamp stage than inverts the signal for differential outputs. Bummer. I have a balanced preamp and amplifier and was hoping to achieved a fully balanced system.
3. The bright side of #2 is that there appear to be only 6 single opamps in the circuit. So, DIP OPA627 can go in without any converters needed.
4. This unit is more complicated than the GDA-600, so apart from doing a recap and changing opamps, I'll probably listen to this more before I mod it much.

More to come.

 

[SoNic67]

The 1702 is balanced inside but summed to reduce distortion, look at the block diagram. To get a true differential mode you would need 2 of those chips per channel and the digital signal "inverted" at input of one.

 

[heyraz]

How does it sound (stock) compared to your modded GDA-600?
I'm pretty impressed with the GDA-700 I got last week. Very open and airy sounding, really fun to listen to.

 

[belgianbrain]

I did the listening test today with my wife.

We both agreed the modified GDA-600 smoked the GDA-700. Modded GDA-600 has much better dynamics and 3D sound staging. The GDA-700 sounded flat in comparison.

The GDA-700 will now undergo surgery to get a full recap, opamp replacement, and have that sound killing LC filter removed from the output. Stay tuned.

 

[SoNic67]

Leave the LC, is not "killing" anything... just the ultrasonic products.

 

[belgianbrain]

...then why did my GDA-600 sound noticeably better with it gone?

 

[belgianbrain]

Parts ordered. I ordered 6x AD797. I decided OPA627 probably isn't worth an extra $100 for 6 opamps. Also, it'll be nice to contrast to my OPA627 equipped GDA-600 when I'm done.

Also ordered a slew of Nichicon and Elna capacitors to replace all capacitors. Ordered Silmic II 100uF and Vishay Roderstein 1837 as opamp bypasses. I'm going with a more conservative approach with this GDA-700 than I did with the GDA-600 and I'll leave the entire digital input section untouched.

Stay tuned.

Sonic, maybe I will leave the output inductors in initially just for shits and giggles. Maybe.

 

[SoNic67]

It's just in your mind xD. Depending on how old you are and how much wax you have in the ears, you will filter more of the top end than that LC group.
But carefull with those AD797. They are power hungry and hot too... Sometimes I prefer the LME49710 just because I think they are less demanding, easier to "tame".

 

[belgianbrain]

Sonic, I'm 39, so I bet I'm well below the average on this board.

Thanks for the tip on the AD797. I read through the data sheet before ordering and looked at the configuration of the GDA-700 and nothing jumped out at me as problematic for the AD797. BUT, you clearly have a lot more experience with this stuff than I do! I will be adding 100uF electrolytic caps to the V- and V+ pins of each AD797. Granted, that's more than the 4.7uF suggested in the datasheet, but I believe in overkill.

As for power hungry, we'll see if the recapped PS survives.

 

[SoNic67]

Yeah, 25uF would be more than enough, 100 is excessive already
The bypass caps of those (0.1uF to ground plane) should be as close as possible of the pins though.

 

[heyraz]

We both agreed the modified GDA-600 smoked the GDA-700. Modded GDA-600 has much better dynamics and 3D sound staging. The GDA-700 sounded flat in comparison.

How long did you listen to the GDA-700 before you decided it was flat by comparison to your modified GDA-600?
I ask because after being stored for a few months by the previous owner, it took a few days for my GDA-700 to "break back in" and open up. Initially I was unimpressed, but a day or two later I was very pleasantly surprised by it's subtle clarity and open soundstage.

But carefull with those AD797. They are power hungry and hot too

I had a similar problem (thermal) using OPA626 or OPA627 in a phono preamp I worked on. They got pretty hot and the sound quality seemed to drop off during use or whenever music passages got loud and complicated. I got the impression the OPA's became noisy when driven hard and my suspicions were confirmed when I checked the datasheet (thermal noise increases with temperature).
I fashioned aluminum fins and attached them to the top of the OPA's with thermal paste to improve heat dissipation. I think it helped, no more ear fatigue after the fins went on.

The aluminum fins look like fat staples sitting on top of the OPA's adjacent to the yellow box caps:

FWIW-I've experimented with a number of OPA's including OPA626 and OPA627 for different projects and can't say the results have been consistent. More often than not, I thought any improvement in clarity OPA626/OPA627 afforded was accompanied by harshness. In fact, I've only used OPA626/627 once because I thought other OPA's sounded better overall depending on the application. Now I have a drawer full of OPA's and am reminded that different is not always better.
In one project I thought a pair of OPA626's on Browndogs was a complete failure probably because there was no place for the heat to go. I ended up chasing my tail trying to fix everything else before I realized the Browndog assembly was running hot.
Synergy has to factor in whenever I voice a component.

Good luck with the GDA-700 mods and keep us posted.

 

[SoNic67]

That's why I stepped back to the LM4562/LME49720 (duals) and LME49710 (single): they are more "civilized" in this respect, and the TO99 can can even have fitted heat sinks (best case). The DIP ones have a little better dissipation anyway, IMO they are just fine as is, maybe just a tad warmer.

 

[belgianbrain]

Recapped the power supply board last night. Brand new Nichicons throughout, and I replaced all 10uF caps with 100uF.

Now on to the digital/analog sections...

 

[belgianbrain]

Quick question, Sonic. I'm almost done with this unit.

If the CLC I see on the output is the reconstruction filter, what is the capacitor network on the second opamp (i.e., C430, 432, 434) doing? The schematic is in our digital documents archive on AK.

http://akdatabase.com/AKview/displayimage.php?album=20&pos=5

 

[SoNic67]

I didn't have this schematic, thanks. I was guessing wrong, they are basically for cutting out any HF digital interferences, in the MHz range, that would go out from the DAC (maybe due to a FCC radiated noise requirement). They are similar with the ones on the SPDIF inputs/output, far out of audio range.
By removing those you will hear just a placebo effect IMO.

BTW, if you look at Parasound D/AC 2000, you will find the exact same LC filters (same 220pF capacitors).

 

[belgianbrain]

Thank you, Sir. Out they go.

 

[SoNic67]

Enjoy the placebo

 

[belgianbrain]

Done! Photos.

 

[belgianbrain]

So, on the main board:

1. Complete recap with Elna Silmic II.
2. Replace 6 op amps with AD797.
3. Each opamp on V+ and V- pin got Elna Silmic 100uF and (on the bottom of the board) bypassed with 0.1uF Vishay Roderstein MKP-1837.
4. Had some extra MKPs, so receiver chip, PCM1702s, and DSP got bypass caps replaced with 0.1uF Vishay Roderstein MKP-1837.
5. CLC networks cut out of output.
6. CLC network cut out of input on Coax 1.

Reconstruction filter on second opamp uses WIMA, so I decided to leave that alone. Those are quality caps.

With the GDA600, I stripped most of the digital input stage and had my SPDIF go directly to receiver chip. However, this design is truly balanced for balanced digital input, so I decided to leave it mostly alone. Only Coax 1 input had some of the superfulous crap removed. That transformer coupling to the receiver chip bugs me though. It does distort the digital input signal when you view it on a scope. We'll see if it survives in the long term.

Anyway, powered it up and it sounds good. My initial reaction is that this will easily match my GDA600, but I'll let it break in for 1-2 weeks while I watch television at night before posting any solid opinions.

 

[belgianbrain]

Sonic, yes, as an engineer I completely understand your comment about the placebo. BUT, I can't deny that I can hear differences in things that I shouldn't (e.g., different expensive high-end boutique coupling caps). So, I try to keep an open mind and I do believe that simpler is better.

I think the rationale behind those input and output filter sections is dubious and probably more related to the numbers game in guaranteeing noise below 0.0000001 what ever.